76 ON THE ORIGIN AND ACTION OF HUMUS. 



first yielded to the young plants as food, in the 

 form of carbonic acid. 



The soluble matter thus acquired by the soil is 

 still capable of decay and putrefaction, and by 

 undergoing these processes furnishes renewed sour- 

 ces of nutrition to another generation of plants; it 

 becomes humus. The cultivated soil is thus placed 

 in a situation exactly analogous to that of forests 

 and meadows ; for the leaves of trees which fall in 

 the forest in autumn, and the old roots of grass in 

 the meadow, are likewise converted into humus by 

 the same influence : a soil receives more carbon in 

 this form than its decaying humus had lost as car- 

 bonic acid. 



Plants do not exhaust the carbon of a soil in the 

 normal condition of their growth ; on the contrary, 

 they add to its quantity. But if it is true that plants 

 give back more carbon to a soil than they take from 

 it, it is evident that their growth must depend upon 

 the reception of nourishment from the atmosphere in 

 the form of carbonic acid. The influence of humus 

 upon vegetation is explained by the foregoing facts 

 in the most clear and satisfactory manner. 



Humus does not nourish plants by being taken up 

 and assimilated in its unaltered state, but by pre- 

 senting a slow and lasting source of carbonic acid, 

 which is absorbed by the roots, and is the principal 

 nutriment of young plants at a time when, being des- 

 titute of leaves, they are unable to extract food from 

 the atmosphere. 



In former periods of the earth's history, its sur- 

 face was covered with plants, the remains of which 

 are still found in the coal formations. These plants, 

 — the gigantic monocotyledons, ferns, palms, and 

 reeds, — belong to a class to which nature has given 

 the power, by means of an immense extension of their 

 leaves, to dispense with nourishment from the soil. 

 They resemble in this respect the plants which we 

 raise from bulbs and tubers, and which live while 

 young upon the substances contained in their seed. 



